How to add a Stroke Layer in a Collection
I think some of the default names for things in Blender are confusing.

A Grease pencil object (yellow line icon in outliner) can be part of a collection. There's a Grease Pencil object called 'GP Object' in the image.
A Grease Pencil object has a Grease Pencil data block (green icon in the outliner). I've called the datablock 'GP' in the image.
That data block has layers. Called 'Lines' and 'Fills' in the image.
Layers have frames (the playhead selects which frame is the current one)
Frames can have strokes and fills (made of points).
With the Grease Pencil object active, you can use the Grease Pencil datablock tab to add/remove layers.